Trigonometry

Ans.not given

@abhisheksingh the sum of the angles of the first and last terms is pi/2.
Same is tru for second abd third term.
Group them together. Multiply divide by 2.
Apply 2cosacosb=cos(ab)+cos(a+b)
You'll see that the "a+b" term nulls down to zero. And the " ab" term will give a good angle.